Liquid detergents for automatic dishwashers have been commercialized since the mid 1980's and have overcome many problems encountered with automatic dishwasher detergent powders. Powdered detergents lose solubility on aging, cake in the dispenser cup especially if the builder used in the formulation is an insufficiently hydrated sodium tripolyphosphate, and dusting is generated by fine particles upon dispensing.
Automatic dishwashing liquids (ADL's) which are structured with thixotropic clays, such as the bentonites, solved many of the powder problems but tended to separate on aging. Additionally, such clay thickened ADL's if not shaken prior to pouring increased significantly in viscosity of the residual liquid so that the last few ounces in the container could not be readily decanted. See U.S. Pat. Nos. 4,116,849 (Leikham); 4,431,559 (Ulrich); and 4,740,327 (Julemont et al.). Formulation of clay structured products with stabilizers, such as polyvalent metal salts, has been disclosed in U.S. Pat. No. 4,752,409 (Drapier et al.).
Gel-structured liquid detergent formulations were thus developed to overcome the deficiency of clay structured products. Such gel detergents do not require shaking and deliver a uniform dosage of each ingredient from the first to last use. When properly formulated to appropriate viscosity, such a gel product could be dispensed almost completely and cleanly from the container. Clear detergent compositions with non-drip properties are obtained by using potassium carbonate and/or potassium pyrophosphate as builders of choice to achieve clarity as described in U.S. No. 4,836,948 (Corring).
Many factors must be considered in selecting a builder for use in detergents as discussed in van Wazer, J. "Phosphorus and it Compounds" Volume 2, Interscience Publishers, Inc. (New York, 1958). These factors include alkalinity, pH, buffering ability, water softening, stability and cost effectiveness. For liquid and gel formulations, phosphate builders which are highly soluble and reversion stable are required.
Tetrapotassium pyrophosphate is a preferred builder for clear liquid and gel detergent formulations because of its solubility characteristics. Tetrapotassium pyrophosphate is, however, deficient in water softening relative to tripolyphosphate because it is a poorer sequestrant. If insufficient pyrophosphate is present such as when used in hard water, highly insoluble pyrophosphate precipitates will form. The ability of pyrophosphate to complex ions lies intermediate between the orthophosphates and the higher polyphosphates.
The use of small amounts of a polyelectrolyte such as sodium polyacrylate have been used in combination with the pyrophosphate builder to improve its effectiveness in hard water as described in EP 239,379. Another method of improving hard water effectiveness for liquid detergents is to use a sodium tripolyphosphate (STPP) builder. However, given the relatively low solubility of STPP in water, its use in homogeneous liquid formulations is limited. A more expensive alternative to achieve a homogeneous gel composition is the use of potassium tripolyphosphate as a builder.
The use of potassium salts in a liquid composition built with STPP allows the STPP within the compositions to attain a higher solubility then in the absence of potassium as disclosed in U.S. No. 3,720,621 (Smeets). The potassium salts are added to these formulations to provide a source of potassium ion.
Another polyphosphate that has been used in the art to produce homogeneous liquid detergent compositions is a glassy phosphate which has an Na.sub.2 O/P.sub.2 O.sub.5 mole ratio of 1.6 to 0. One such glassy polyphosphate is sodium hexametaphosphate. Hexametaphosphate has an Na20/P.sub.2 O.sub.5 ratio of about 1.1. Its chemical formula is EQU Na.sub.n +2P.sub.n O.sub.3n+1
wherein n is 13 to 18 as described in Monsanto Chemical Company's product sheet entitled "Sodium hexametaphosphate" (June 1983), Publication No. 9047.
Other glassy polyphosphates are infinitely soluble in water, but they are not useful as builders in liquid detergents partially because of hydrolysis forming crystalline sodium phosphate which comes out of the solutions upon standing. See van Wazer, J. "Phosphorus and its Compounds", Volume 2, Interscience Publishers, Inc. (N.Y., 1958), page 1762.
Sodium trimetaphosphate has been described as a builder useful for making a homogeneous liquid detergent. Its ability to complex metal ions is not sufficiently strong to be of commercial value; however, when the trimetaphosphate anion is hydrolyzed in a strongly alkaline solution, sodium tripolyphosphate is formed. van Wazer, J., "Phosphorus and its Compounds", Volume 1, Interscience Publication, Inc. (N.Y., 1958) Pages 456 , 462, 641 and 704. Moreover, it is known that to prepare a mixed salt (Na/K) tripolyphosphate, sodium trimetaphosphate is hydrolyzed with caustic potash to form sodium potassium tripolyphosphate as follows: ##STR1##
The foregoing reaction is hydrolyzed in a temperature range of from 45.degree. C. to 70.degree. C. In performing this reaction, the maximum solids content should not exceed 38% because sodium ions are most detrimental to the solubility of sodium potassium tripolyphosphate. Therefore, sodium ions should be avoided while potassium salts should be chosen. Monsanto Chemical Company, Marketing Technical Service Information Bulletin (April, 1990).
U.S. No. 5,053,158 (Dixit et al.) combines a builder salt selected from the polyphosphates including alkali metal tripolyphosphate, alkali metal pyrophosphates and alkali metal metaphosphate, with silicate, alkali metal hydroxide, chlorine bleach stable organic detergent active, thickening agent and a long chain fatty acid or its metal salt. According to Dixit et al. the thickening agent and fatty acid components must be present in order to maintain the stability of the compositions. The sodium and potassium ions must be present in a K/Na weight ratio of from about 1/1 to about 45/1. Potassium salts are used in Dixit et al. to solubilize the builders and provide the claimed homogenity.